The term "lipid" generally denotes a heterogeneous group of substances, associated with living systems, which have the common property of insolubility in water but solubility in non-polar solvents such as hydrocarbons or alcohols. Included in the group are the oils and fats of our diet together with the so-called phospholipids associated with cell membranes. These substances have in common that they are esters of long-chain fatty acids.
Monocarboxylic, aliphatic fatty acids are the structural components common to most of the lipids that interest food chemists, and many of the properties of food lipids can be accounted for directly in terms of their component fatty acids. Almost without exception the fatty acids that occur in foodstuffs contain an even number of carbon atoms in an unbranched chain, e.g. lauric and dodecanoic acid. Besides the saturated fatty acids, of which lauric acid is an example, unsaturated fatty acids having one, two, or sometimes up to six double bonds are common.
Alpha-linolenic acid (systematically all-cis-9,12,15-octadecatrienoic acid) has the structure: EQU CH.sub.3 --CH.sub.2 --CH.dbd.CH--CH.sub.2 --CH.dbd.CH--CH.sub.2 CH.dbd.CH--(CH.sub.2).sub.7 --COOH
Gamma-linolenic acid is a less common isomer with double bonds at the 6-, 9- and 12-positions.
The system used for the identification of double-bond positions will be apparent by comparison of the structure with the systematic name. The structure of a fatty acid can be indicated by a convenient shorthand form giving the total number of carbon atoms followed by a colon and then the number of double bonds with the position of the double bonds given after the symbol .DELTA.. Thus for example .alpha.-linolenic acid would be written simply as 18:3.DELTA.9,12,15.
The oils and fats are obviously the lipids that most interest the food chemist. These consist largely of mixtures of triglycerides, i.e. esters of the trihydric alcohol glycerol (propane-1,2,3-triol), and three fatty-acid residues which may or may not be identical. "Simple" triglyceride molecules have three identical fatty-acid residues while "mixed" triglycerides have more than one species of fatty acid. Thus a naturally occurring fat will be a mixture of quite a large number of mixed and simple triglycerides. It is important to remember that organisms achieve a desirable pattern of physical properties for the lipids of, for example, their cell membranes or adipose tissue by utilizing an appropriate, and possibly unique, mixture of a number of different molecular species, rather than by utilizing a single molecular species which alone has the desired properties, as is the usual tactic with proteins and carbohydrates.
Fats and oils can be viewed in terms of their component triglycerides. The first descriptions of the glyceride structure of fats assumed that all their component triglycerides were simple. Thus a fat containing palmitic (hexadecanoic), stearic (octadecanoic), and oleic (cis-octadec-9-enoic) acids would be a mixture of the three triglyceride species tripalmitin, tristearin, and triolein. The first attempts to separate the component glycerides of fats, by the laborious process of fractional crystallization from acetone solutions at low temperatures, made it clear that much greater numbers of species of triglycerides occurred than would be expected from this simple concept. Fats and oils became recognized as clearly defined mixtures of mixed and simple triglycerides.
The fatty acids, in the form of the triglycerides of the dietary fats and oils, provide a major proportion of our energy requirements as well as, when in excess, contribute to the unwelcome burden of superfluous adipose tissue that so many of us carry. In recent years we have begun to appreciate that certain dietary fatty acids have a more particular function in human nutrition. Rats fed a totally fat-free diet show a wide range of acute symptoms affecting the skin, vascular system, reproductive organs, and lipid metabolism. Although no corresponding disease state has ever been recorded in a human patient, similar skin disorders have occurred in children subjected to a fat-free diet. The symptoms in rats could be eliminated by feeding linoleic or arachidonic acids (which in consequence became known for a time as vitamin F), and it is generally accepted that 2-10 g of linoleic acid per day will meet an adult human's requirements.
The identification of these two "essential fatty acids" in the 1930s preceded by some 25 years their identification as precursors of a group of animal hormones, the eicosanoids. Although animal tissues are unable to synthesize either of these two fatty acids, they readily convert the C.sub.18 acid to the C.sub.20 acid.
The many different eicosanoids all have similar structures. The reasons for the stringent requirements for the positions of the double bonds in essential fatty acids are clearly evident from the biosynthesis of prostaglandin E.sub.2 from linoleic acid.
Other eicosanoids vary in the degree of reduction of the ring oxygens and presence of double bonds in the chain. Details of their numerous physiological activities are still accumulating in the scientific literature, but they are best known for their involvement in inflammation and the contraction of smooth muscle.
There are indications from studies of Eskimos that it is the high levels in their diets of certain polyunsaturated fatty acids (n-3 fatty acids which are abundant in fish oils) that are responsible for the remarkably low incidence of arterial disease in a population that appears to break all the usual nutritional rules. Fish oils are rich in fatty acids such as eicosapentaenoic acid (20:5.DELTA. all cis-5,8,11,14,17) and docosahexaenoic acid (22:6.DELTA. all cis-4,7,10,13,16,19). As seen from their structural formulae, these fatty acids are characterized by having a double bond in the n-3-position, i.e. at the third carbon atom when counting from the methyl end of the hydrocarbon chain. The nomenclature of n-3 is equivalent to the old .omega.-3 designation. This means that a quite distinct set of eicosanoids are derived from them compared with those from the so-called n-6-series. Prostaglandins synthesized from n-6 fatty acids are generally more active than those from n-3-fatty acids in promoting the formation of the blood clots that are involved in thrombosis. It remains to be seen whether these observations will lead to useful modifications of our diet or to changes in clinical practice.
For many years, it has been known that levels of thromboxane A.sub.2, prostacyclin and PGE.sub.2 (collectively "series 2 prostanoids") are elevated in endotoxemia and play a crucial role in septic and endotoxic shock, particularly in endotoxic shock caused by lipopolysaccharides from gram-negative bacteria such as E. coli. These same metabolites (series 2 prostanoids) have been shown to increase in a variety of other disease and stress states. Moreover, there is an imbalance between series-1 and series-2 prostaglandins in disease states such that the harmful series-2 prostaglandins predominate. Series 2 prostaglandins are formed from arachidonic acid (20:4n-6) which is derived from the n-6 fatty acid linoleic acid (18:2n-6) by enzymatic desaturation and elongation reactions.
Leukotriene B.sub.4 (LTB.sub.4) is a metabolite of arachidonic acid formed via a lipooxygenase enzyme. LTB.sub.4 is a potent chemotactic agent for neutrophils and has been shown to stimulate neutrophils to secrete large quantities of potentially injurious mediators in inflammatory diseases. The use of n-3 fatty acids will regulate the intensity of n-6 prostaglandins and leukotriene biosynthesis since excess eicosanoid production can cause pathophysiology.
In the last few years, there have been a number of attempts to alter the relative supply of dietary n-3/n-6 fatty acids to modify the eicosanoid synthesis pathway and shift the proportions of series 1, series 2 and series 3 eicosanoids to produce a more desirable health status. It is known that both n-3 and n-6 types of fatty acids can be metabolically elongated and desaturated, however, the body cannot change the position of the double bonds; therefore, n-3 fatty acids cannot be converted to n-6 fatty acids and visa versa. Since each type of eicosanoid comes from a different family of fatty acids (e.g., n-3, n-6, n-9), diet modification is a promising course to modulate tissue eicosanoid biosynthesis.
U.S. Pat. No. 4,752,618 ("'618 Patent"), issued Jun. 21, 1988, the disclosure which is incorporated herein by reference, was one of the earliest references which discloses diet modification for treatment of disease. The '618 Patent describes the treatment of infection in patients through reducing the amount of n-6 fatty acids in the diet (particularly reduction of linoleic acid) by replacing a portion of the n-6 fatty acids with n-3 fatty acids. The optimum source of n-3 fatty acids disclosed in the '618 Patent is fish oil, e.g., menhaden oil. This dietary modification leads to the production of a larger proportion of series "3" prostanoids in place of series "2" prostanoids than normally is obtained from standard diets. Although the series "3" prostanoids, and the attendant reduction of series "2" prostanoids, has substantial beneficial effects, in some circumstances, particularly in the treatment of endotoxic shock, replacement of series "2" prostanoids with series "1" rather than the series "3" prostanoids might be even more beneficial. Series "1" prostanoids have already been shown to provide a certain amount of protection in endotoxic lung injury and traumatic shock.
The synthesis path for forming the series "1" prostanoids is from linoleic acid (18:2n6) to gamma-linolenic acid (18:3n6 or GLA) to dihomogamma-linolenic acid (20:3n6) to the series "1" prostanoids.
The following represents the metabolic pathway of linoleic acid to series "1" and "2" prostaglandins. ##STR1##
Dihomogamma-linolenic acid competes with arachidonic acid (20:4n6), for the enzyme cyclooxygenase. Cyclooxygenase is a critical enzyme in the formation of both the series "1" and series "2" prostanoids. When GLA is formed endogenously substantially all the gamma-linolenic acid is made into arachidonic acid, the precursor of the series "2" prostanoids. Accordingly, one could modify the diet to contain relatively high levels of gamma-linolenic acid in order to skew the prostanoid synthesis pathway to preferentially increase the production of series "1" prostanoids.
In a paper by Hirschberg et al., "The Response to Endotoxin in Guinea Pigs After Intravenous Blackcurrant Seed Oil," Lipids 25, 491-496 (1990) it is disclosed that high levels of blackcurrant seed oil, an oil rich in gamma-linolenic acid, was supplied as part of a parenteral diet to guinea pigs, who were then challenged with endotoxin. The results were somewhat disheartening; the gamma-linolenic acid provided no better protection (and possible worse systemic results) against endotoxin shock than did the classic lipid diet with soybean oil, a diet high in linoleic acid.
However, a recent study by Karlstad et al. JPEN 1992; 16(1):215 disclosed the measurement of the levels of dihomogamma-linolenic acid in the blood after the addition of 0, 2.7%, 4.4% and 6.1% gamma-linolenic acid to a parenteral diet. The authors found that for 4.4% and 6.1% gamma-linolenic acid enrichment, there was a 4-5 fold increase in the plasma dihomogamma-linolenic/arachidonate ratio. The increase in plasma dihomogamma-linolenic acid should lead to the production of more series "1" prostanoids.
The results of the Karlstad et al. and Hirschberg studies can be interpreted to mean that, beyond a certain level, dietary gamma-linolenic acid is not utilized properly. It may be that excess gamma-linolenic acid may be formed into arachidonic acid, leading to series "2" prostanoid buildup. Accordingly, one problem is how to achieve a higher level of dihomogamma-linolenic acid in plasma and tissues without parallel buildup of arachidonic acid.
It has been theorized that a structured lipid containing a medium chain fatty (C.sub.6 -C.sub.12) acid residue may provide improved absorption of other fatty acids attached to the structured lipid. A recent paper by Jensen A.J.C.N. Suppl. no. 62; 1992 disclosed that a structured lipid containing medium chain fatty acid residues and long chain fatty acid residues (n-3 fatty acids from fish oil) are absorbed faster by the body than the physical mixture of the same fatty acids. There is no suggestion or teaching that a specific structured lipid would be useful to modify the prostanoid synthesis pathway.
U.S. Pat. No. 4,906,664 discloses a method of treating patients with cancer through administering a diet containing a structured lipid of the formula: ##STR2## where one of R.sub.1, R.sub.2 and R.sub.3 is a medium-chain fatty acid, and a second one of R.sub.1, R.sub.2 and R.sub.3 is an .omega. fatty acid, and the third one of R.sub.1, R.sub.2 and R.sub.3 is selected from the group consisting of hydrogen, hydroxyl, short, medium and long-chain fatty acids. This reference does not suggest or disclose the specific structured lipid of the instant application.
European Patent Application Number 87114297.2 discloses a triglyceride having a C.sub.8 to C.sub.14 fatty acid residue at the 2-position of the triglyceride and a residue of C.sub.18 or higher fatty acids at the 1 and 3 position thereof. There is no suggestion or disclosure of the specific structured lipids of the instant invention nor the benefits that can be realized by feeding the structured lipids of this invention.
International Application No. PCT/DK 89/00239 filed Oct. 10, 1989 discloses the triglycerides 2-docosahexaenoyl!-1,3-di(octanoyl/decanoyl) glycerol for nutritional compositions for enteral or parenteral purposes, especially as breast milk replacers.
International Application No. PCT/DK 89/00237 filed Oct. 10, 1989 discloses the triglycerides 2-arachidoyl-1,3-di(octanoyl/deconoyl) glycerol and the use of these materials in nutritional products.
International Application Number PCT/US89/01364 with a publication number of WO 89/09596 discloses a transesterification product of a mixture of fatty acids and triglycerides which include dairy fat as a primary component. A method of nutritional support using this composition is also disclosed.
International Application Number EP 421,867 discloses the production of structured lipids enriched in gamma-linolenic and/or stearidonic fatty acids. The process comprises hydrolysing a mixture of glycerides or the fatty material containing them with a lipase having specificity such as not to hydrolyse the ester bond of the gamma-linolenic and stearidonic fatty acids esterified in position 1, 2 or 3 and recovering the non-hydrolysed residue from the enzymatic reaction by separating the fatty acids produced.
Canadian Patent Application 2000391 with a WPI Accession Number of 90-139962/19 discloses the triglyceride 2-(alpha-linolenoyl)/gamma-linolenoyl)-1,3-di (octanoyol/decanoyl) glycerol as useful in nutritional compositions. It is suggested that these triglycerides are useful as components in nutritional compositions. The fatty acids are essential for control of tonus of smooth-muscle cells in the blood vessels or the tonus of the smooth muscle cells in the lungs and thus are useful in the control of respiratory distress. This reference does not suggest or disclose the specific structured lipids of this invention nor the methods of using them.
U.S. Pat. No. 4,528,197 discloses a method of enhancing protein anabolism in a hypercatabolic mammal, the method comprising parenterally administering an emulsion of triglycerides which, on hydrolysis, yields both long chain fatty acids and medium chain fatty acids.
U.S. Pat. No. 4,871,768 discloses a synthetic triglyceride comprising a glycerol backbone having three fatty acids attached thereto, said fatty acids being selected from a first group consisting of .omega.-3 fatty acids, and a second group consisting of caprylic acid, capric acid and mixtures thereof. This patent also discloses a method for minimizing the effects of infection and minimizing the effects of subsequent infection by administering a diet containing 10 to 80% by weight of an oily fraction, said oily fraction being the aforementioned fatty acid.
U.S. Pat. No. 4,701,469 discloses triglycerides of the formula. ##STR3## wherein R represents an acyl fragment of a polyunsaturated fatty acid containing 18 to 22 carbon atoms, the acyl fragment being capable of being oxidized, however, R cannot represent the acyl fragment of eicosatetrayn-5, 8, 11, 14-oic acid, and wherein n represents an integer varying from 2 to 16; a process for their preparation, their dietetic and therapeutic applications and compositions containing them.
None of these references either suggest or disclose a structured lipid of the formula: ##STR4## wherein
(1) at least one of R.sub.1,R.sub.2 or R.sub.3 is a fatty acid residue esterified to glycerol and selected from the group consisting of gamma-linolenic acid, dihomogamma-linolenic acid, and active derivatives thereof;
(2) a second of R.sub.1, R.sub.2 or R.sub.3 is a fatty acid residue esterified to glycerol and selected from the group consisting of C.sub.18 -C.sub.22 n-3 fatty acids and C.sub.6 -C.sub.12 fatty acids and active derivatives thereof; and
(3) a third of R.sub.1,R.sub.2 or R.sub.3 is a fatty acid residue esterified to glycerol and selected from the group consisting of C.sub.6 -C.sub.12 fatty acids and active derivatives thereof.
Further, these references fail to suggest or disclose a method of modulating metabolic response to trauma and disease states in patients through administering the structured lipid of this invention.
One benefit of this invention over the prior art is that a structured lipid containing a GLA or DHGLA residue and a medium chain fatty acid residue (C.sub.6 -C.sub.12) will increase the incorporation of the GLA or DHGLA into tissues and thereby beneficially modify eicosanoid biosynthesis. Medium chain fatty acids in the structured lipid also provide additional fat calories and increase the absorption and clearance of the structured lipid so that the reticuloendothelial system is not blocked with an overabundance of long chain triglycerides. More importantly, medium chain fatty acids do not act as substrates for eicosanoid synthesis. Accordingly, one aspect of the present invention is concerned with a structured lipid which modifies eicosanoid synthesis in a positive manner to produce more series "1" eicosanoids. Another aspect of the invention relates to a physical blend of structured lipids. The first structured lipid contains gamma-linolenic acid and/or dihomogamma-linolenic acid and C.sub.6 -C.sub.12 fatty acid residues and a second structured lipid which contains n-3 fatty acid residues and C.sub.6 -C.sub.12 fatty acid residues.
An additional aspect of the invention is to provide a method of treating disease and stress states using the specific structured lipid of the invention. These and other features of the invention will be apparent from the following description and the claims.